Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture comprised of a polyester or polyether polyol, a polyisocyanate, a surfactant, catalysts, and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure.
Polyurethane foams with varying density and hardness may be formed. Hardness is typically measured as IFD ("indentation force deflection") or CFD ("compression force deflection"). Tensile strength, tear strength, compression set, air permeability, fatigue resistance, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing. Among other things, polyurethane foams are widely used as energy absorbing cushions and fillers in the packaging industry.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, polyisocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and airtight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
U.S. Pat. No. 4,777,186 to Stang, et al., describes a method of foaming in a pressurized chamber held above atmospheric pressure (i.e., in the range of about 0.5 to 1000 psig). In addition to the gases emitted during foaming, additional gases may be introduced into the chamber to maintain the elevated pressure during foaming. The resulting foams have a higher ILD to density ratio than those previously known to the art.
Those of skill in the packaging industry characterize dynamic shock cushioning characteristics ("energy absorption") of materials by developing "drop curves" or plots of deceleration versus static load in accord with ASTM D 1056. A foam is cut to a predetermined size, typically 8".times.8".times.2" (thickness) and positioned on an impact surface. A dropping platen with an adjustable load is dropped onto the sample. Instrumentation measures both the peak impact deceleration ("G-level") and impact velocity as the platen deflects the cushion. The impact velocity is checked to be within tolerances, and the peak "G-level" is recorded. The impact velocity corresponds to a "free fall drop height," which is measured in order to compensate for the effect of friction in the dropping apparatus, but the corresponding free fall drop height is typically reported as if it were measured physically. The most commonly used free fall drop height is 24 inches. The platen, with the same static loading, is dropped on the same cushion five times. The static loading is calculated by dividing the mass of the platen by the surface area of the foam sample. Each drop is separated by about one minute. A new cushion sample is used, and a sequence of five drops is performed for another static loading, usually determined by the experience of the operator during the test. The process is repeated until enough data points have been gathered to draw a representative curve. The average of the second through fifth drops is commonly reported as the average "G-level" for each static loading. Lower "G-levels" indicate greater energy absorption by the foam, or less shock felt by the platen or what would be the packaged object in packaging applications. Prior art packaging materials using a two-inch sample thickness and a 24-inch "free fall drop height" generally yield G-levels above 60 G at a 1 psi static loading.
An object of the present invention is to produce energy absorbing foams with "drop curves" substantially improved over those previously obtained in the prior art. Where foams with improved "drop curves" are used in packaging applications, either less foam is required for the same energy absorbing protection, or the foam may be used to package heavier objects than previously possible.